The blue tubs are in my opinion at least 30 to 40gal tubs not really sure what the gallons are due to not having that info on the side of the tubs, as for any filtration or overfills I do not have any, I use plants as filtration and in my area around summer time we really slow down on the rain fall, so my water levels stay low.. I usually have problems like birds and lizards drowning and an occasional squirrel not making it out so i try to place objects like wood limbs so that critters can climb out.. Plants I use are Water Hyacinth, Naja Grass, mosses and Anacharis, I place my tubs in full shade with just a bit of morning sun, my two larger ponds are basically the same way..